Starting circuit for a synchronous machine having an electronic commutator

ABSTRACT

A starting circuit for a synchronous machine having controlled semi-conductors in the individual winding circuits of the machine is disclosed in which at least one auxiliary circuit connected in parallel to the windings is used. The auxiliary circuit comprises a capacitor which prior to operation of the circuit, is charged to a voltage opposite to the conducting voltage of the winding thyristors. An auxiliary thyristor connected to conduct in the same direction as the winding thyristors is connected in series with the capacitor and a diode of opposite polarity is connected in parallel to the auxiliary thyristor. When a winding thyristor is to be cut off the auxiliary thyristor is triggered to conduct thereby causing the capacitor to discharge through the conducting winding thyristor thereby placing a negative cut off voltage on this thyristor and cutting off any further conduction of the thyristor.

United States Patent 1191 Hubner 1 Jan. 15, 1974 1 1 STARTING CIRCUIT FOR A SYNCHRONOUS MACHINE HAVING AN [73] Assignee: Siemens Aktiengesellschaft, Munich,

Germany [22] Filed: Aug. 17, 1972 [21] App1.No.: 281,556

[30] Foreign Application Priority Data 7/1972 Bcdford 318/138 Primary ExaminerBcrnard A. Gilheany Assistant Expminen-W. E. Duncanson, Jr. Att0rneyHugh A. Chapin [57] ABSTRACT A starting circuit for a synchronous machine having controlled semi-conductors in the individual winding circuits of the machine is disclosed in which at least one auxiliary circuit connected in parallel to the windings is used. The auxiliary circuit comprises a capacitor which prior to operation of the circuit, is charged to a voltage opposite to the conducting voltage of the winding thyristors. An auxiliary thyristor connected to conduct in the same direction as the winding thyristors is connected in series with the capacitor and a diode of opposite polarity is connected in parallel to the aux iliary thyristor.

When a winding thyristor is to be cut off the auxiliary thyristor is triggered to conduct thereby causing the capacitor to discharge through the conducting winding thyristor thereby placing a negative cut off voltage on this thyristor and cutting off any further conduction of the thyristor.

3 Claims, 9 Drawing Figures l e "P11 1 I 3* PATENTEDJAN15I974 3;786,326

' SHEETIUFZ PATENTED JAN 1, 5 I974 SHEET 2 OF 2 Fig. 2

STARTING CIRCUIT FOR A SYNCIIRONOUS MACHINE HAVING AN ELECTRONIC COM-MUTATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is concerned with a starting circuit for a synchronous machine fed with an impressed d-c current, and having a commutator of controlled semiconductor rectifiers (thyristors) in the individual winding circuits.

2. Description of the Prior Art In operating synchronous machines of the type involved in this invention, called converter motors, the reactive power for the converter is made available by the machine itself. This occurs provided the customary design of the machine permits the machine to rotate at a sufficient speed of rotation above approximately percent of its nominal speed and provided, upon the firing of each successive thyristor, the machine voltage is sufficient to commutate the machine current from the winding through which the current is flowing, to the next winding. Below this machine speed the machine voltage is too low to extinguish the prior currentcarrying thyristor when the next thyristor is cyclically fired, so that a commutation short circuit would occur unless special starting aids are provided. The known starting aids are expensive and depend on various parameters, such as, for example, the line frequency, machine frequency, etc.

It is an object of the invention to create a startingcircuit of the kind mentioned above, which is of simple design and operates independently of such parameters.

SUMMARY OF THE INVENTION According to the invention, the stated problem is solved by providing at least one auxiliary circuit with the commutator which comprises a capacitor connected with an auxiliary voltage source to make available a charging voltage and to cover the chargereversal losses. Additionally a rectifier arrangement is connected in series therewith and consists of a diode and a parallel connected, reverse polarity auxiliary thyristor connected to the'discharging circuit of the capac itor. The discharging circuit only contains the currentcarrying winding circuit, whereby the winding circuit is controlled and forced into conduction in synchronism with one thyristor of the commutator.

After the machine has reached sufficient speed so thatthe machine voltage assures proper commutation, the starting circuit can be disconnected, preferably in reliance upon the speed. According to the invention the starting circuit is here based on the use of additive extinction, known from the control of choppers. This principle is applicable with a reduced expenditure for circuitry, because elements ofthe converter motor are used over again as parts of an additive extinction circuit.

BRIEF DESCRIPTION OFTI'IE DRAWINGS Further details of the invention will be explained more fully in the following with the aid of two examples of embodiments shown in a simplified manner in the drawings in which there is shown in:

FIG. la, a four-pulse converter motor,

FIG. lb, a six-pulse converter motor, and in FIG. 2a to 2g, current and voltage curves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to FIG. la, an intermediate d-c circuit with a smoothing choke Lg, having the stator windings of the convertor motor M connected to it by the respective thyristors, pl to p4, is also connected to an a-c fed, single-phase, half-controlled rectifier bridge G. Connected in shunt with the winding circuits is an auxiliary circuit comprising a capacitor C in series with a diode D and an auxiliary thyristor p connected in reverse polarity and in parallel to the diode. Also connected between the auxiliary thyristor p and the capacitor C is an auxiliary voltage source U which prior to operation charges the capacitor to a charging voltage U,, and thereby compensates for the charge-reversal losses during starting. The stator windings of the converter motor have a stray inductance L 0' The load or motor current, 1,, is held essentially constant by the smoothing choke L At operating speeds of n 0.1 to 0.2 n the converter motor is commutated in the known manner by the machine voltage. At lower operating speeds the auxiliary circuit is connected in a manner not shown. It may be assumed, that the thyristor pl is in its conductive condition and that the thyristor p2 will be fired with an appertaining firing control circuit. If the auxiliary thyristor p is triggered simultaneously with the firing of the thyristor p2 at the pointin time a,( FIG. 2a the capacitor C, which is charged by the auxiliary voltage source U to the negative voltage U (FIG. 2e), discharges through the current-conducting thyristor pl and the fired auxiliary thyristor p. According to FIG. 2c, a sinusoidalcounter-currentlc flows with an amplitude 3e U V L U /C. This current, through appropriate proportioning of the voltage U and the capacity C, is larger than the charging current Jg. The countercurrent Jc reduces the motor current J M (FIG. 1 a) until the latter becomes zero at the point in time a so that the thyristor pl is cut off. At that time a, (FIGS. 2e, f) the negative voltage U remaining at the capacitor C acts as a negative cut-off voltage at the thyristor pl (FIG. 2]). The charge on the capacitor C is now reversed through the conducting of the auxiliary thyristor and the constant load current Jg (FIG. 2a) until, according to FIG. 2e, the voltage U becomes zero at the time a The time interval between a, and a, corresponds to the recovery time of the commutator thyristor to be extinguished (for instance, p1). From the time a; on, the capacitor C is charged in an increasingly positive manner. According to FIG. 2b, the load current, Jg, in the machine winding associated with the thyristor p2, is also being built up with the increasing capacitor voltage, until at the point in time a, the machine current J reaches the value J g and the capacitor current Jc becomes zero.

In this explanation, the counter-voltage of the converter motor at the very low starting speeds is neglected as a first approximation. For the shaded voltage-time areas (FIG. 2e) one has F, F, I u dt L a Jg. From the point in time a 4 on, the voltage U of the positively charged capacitor C reverses through the diode D, the machine winding and the series-connected thyristor p2, until at time a5, it has the polarity and magnitude U necessary for extinguishing the currentcarrying thyristor. The losses occurring during the charge reversal are covered here by the auxiliary voltage source U which charges the capacitor C to the voltage U at the initial starting of the motor.

The charge-reversal losses, U a, U 01 U due to the necessary reserve of voltage-time area from a2 to a3 (recovery time of the thyristor to be extinguished) are here neglected.

The entire commutation time, including the reversal process (oz to a is 1- 21r L C.

The repetition frequency of the commutations is f, fp, where f is the motor frequency and p the number of pulses of the inverter.

For the required torque uniformity, the currentless time of the motor (1,, 0) should be as short as possible. Hererthe equation m "fp) ppl With J U V L a /C,' the magnitude of the voltage U and the capacity C can be determined therefrom.

In order to eliminate the possibility of objectionable, currentless state of the motor during commutation, and to obtain a torque, even though smaller at the shaft during the commutation, it is advantageous to provide, in a six-pulse converter circuit according to FIG. lb, two auxiliary circuits 1 and 2, and specifically, one for each half-bridge pl to p3 and p4 to p6 of the machine winding circuits. By this connection, during commutation in one half of the bridge, the load current in the other half of the bridge can continue to flow unimpeded.

The d-c voltage source may also be a battery or an uncontrolled or, as shown, fully controlled rectifier bridge.

In the foregoing, the invention has been described in reference to specific exemplary embodiments. It will be evident, however, that variations and modifications, as well as the substitution of equivalent constructions and arrangements for those shown for illustration, may be made without departing from the broader scope and spirit of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.

What is claimed is:

l. A starting circuit for a synchronous machine hav ing an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits of which the first thyristor is conducting and the second is not, said circuit comprising:

at least one auxiliary circuit connected in parallel to the winding circuits comprising capacitance means.

an auxiliary voltage means removably connected to the capacitance means to charge the capacitance means, prior to the operation of the auxiliary circuit, to a voltage opposite to the conducting voltage of the semi-conductor rectifier in the individual winding circuits a rectifier circuit connected in series with the capacitor comprising an auxiliary control means having the same conducting polarity as the rectifiers in the windings, to conduct upon the triggering of the second of the controlled rectifiers in the winding circuits, and a diode of opposite polarity to the auxiliary thyristor, connected in parallel to the auxiliary thyristor, and

control means to trigger the second thyristor and the auxiliary thyristor whereby the capacitance means discharges through the first thyristor and the auxiliary thyristor, thereby applying a negative voltage to the thyristor to cut off the first thyristor.

2. A starting circuit for a synchronous machine having an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits as in claim 1 in which the capacitance means comprises a capacitor, and the auxiliary voltage means is a battery.

3. A starting circuit for a synchronous machine having an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits as in claim 1 in which at least two auxiliary circuits are used with the machine winding circuits, one for each half bridge of the machine winding circuits. 

1. A starting circuit for a synchronous machine having an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits of which the first thyristor is conducting and the second is not, said circuit comprising: at least one auxiliary circuit connected in parallel to the winding circuits comprising capacitance means an auxiliary voltage means removably connected to the capacitance means to charge the capacitance means, prior to the operation of the auxiliary circuit, to a voltage opposite to the conducting voltage of the semi-conductor rectifier in the individual winding circuits a rectifier circuit connected in series with the capacitor comprising an auxiliary control means having the same conducting polarity as the rectifiers in the windings, to conduct upon the triggering of the second of the controlled rectifiers in the winding circuits, and a diode of opposite polarity to the auxiliary thyristor, connected in parallel to the auxiliary thyristor, and control means to trigger the second thyristor and the auxiliary thyristor whereby the capacitance means discharges through the first thyristor and the auxiliary thyristor, thereby applying a negative voltage to the thyristor to cut off the first thyristor.
 2. A starting circuit for a synchronous machine having an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits as in claim 1 in which the capacitance means comprises a capacitor, and the auxiliary voltage means is a battery.
 3. A starting circuit for a synchronous machine having an impressed d-c input and a commutator having at least two thyristors in the individual winding circuits as in claim 1 in which at least two auxiliary circuits are used with the machine winding circuits, one for each half bridge of the machine winding circuits. 